Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.5606v&link_type=abstract
American Astronomical Society, 181st AAS Meeting, #56.06D; Bulletin of the American Astronomical Society, Vol. 24, p.1214
Astronomy and Astrophysics
Astronomy
Scientific paper
Stellar evolution through the asymptotic giant branch (AGB) and planetary nebula (PN) phases is examined with particular emphasis on stellar mass loss. The Mount Stromlo Stellar Structure Code has been used to produce 22 stellar evolutionary sequences, complete from the main sequence to the white dwarf regime. The models correspond to initial masses of 0.89, 0.945, 1.0, 1.5, 2.0, 2.5, 3.5, and 5.0 M\s, and metallicities 0.016, 0.008, 0.004, and 0.001 Z\s. Mass loss during the AGB phase has been incorporated into the calculations following the empirical relationship between mass loss rate and pulsation period defined by Galactic AGB stars. The calculations show that the overall time-dependent behaviour of the mass loss rate is significantly modulated during any helium shell flash cycle, and that the AGB phase is terminated by severe mass loss over the last 2--3 helium shell flash cycles. The maximum AGB luminosities predicted are in excellent agreement with those observed for Magellanic Cloud AGB stars. [2mm] No effort was made to control the phase of the helium shell flash cycle at which the AGB stage was terminated. From these results, it is estimated that at least 40% of observed PN nuclei can be classified as helium burners, and this will have a significant influence on the PN luminosity function that may be derived for various systems. [2mm] The AGB calculations also indicate that one is likely to observe normal M-, S-, or C-type AGB stars with hollow circumstellar shells. Time-dependent hydrodynamic models of the winds around such stars show that circumstellar matter can flow back towards the star during the luminosity dip experienced immediately following a flash event. Multiple shell planetary nebulae may also be the result of this type of behaviour, though the hydrodynamic calculations have not been extended this far as yet. [2mm] Future work is hoped to constrain the mass loss behaviour through the transition from AGB star to PN\@.
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